Porous lithium garnet makes effective solid electrolyte

Lithium-ion batteries store a lot of energy in a small space, making them the energy source of choice for mobile electronic devices: mobile phones, laptops, e-bikes and electric cars are all powered by such batteries. Now, in a paper in Advanced Energy Materials, researchers at ETH Zurich in Switzerland report developing a novel type of lithium-ion battery that, unlike conventional ones, consists entirely of solid chemical compounds and is non-flammable.

Conventional lithium-ion batteries are not without their dangers: mobile phone batteries have exploded several times in the past, resulting in injuries, and only six months ago an entire row of houses burned down in the Swiss town of Steckborn on Lake Constance. The blaze was caused by a model-making battery that allegedly caught fire due to being charged improperly.

In conventional lithium-ion batteries, as well in most other batteries, the positive and negative electrodes are made of solid conductive compounds, with charges moving between these electrodes through a liquid or gel electrolyte. If you charge such a battery improperly (overcharging) or leave it sitting out in the sun, the liquid electrolyte can ignite or the gel can swell up.

This is not the case with solid-state batteries, which are currently in development in research laboratories worldwide: in these types of batteries, both the electrodes and the intermediary electrolyte are made of solid materials. "Solid electrolytes do not catch fire even when heated to high temperatures or exposed to the air," explains Jennifer Rupp, who, as professor of electrochemical materials at ETH Zurich, is leading the development of this new type of battery.

One of the challenges in developing solid-state batteries is connecting the electrodes and electrolyte in such a way that the charges can circulate between them with as little resistance as possible. The ETH researchers have now developed an improved electrode-electrolyte interface.

In the laboratory, they constructed a sandwich-like battery featuring a layer of a lithium-containing compound (lithium garnet), which acts as a solid electrolyte between the two electrodes. Lithium garnet has one of the highest conductivities for lithium ions of any known material.

"During production, we made sure that the solid electrolyte layer obtained a porous surface," says Jan van den Broek, a master's student in Rupp's group and one of the authors of the study. The researchers then applied the negative electrode in a viscous form, allowing it to seep into the electrolyte’s pores. Finally, they heated the battery at 100°C.

"With a liquid or gel electrolyte, it would never be possible to heat a battery to such high temperatures," says van den Broek. Thanks to the trick with the pores, the researchers were able to significantly enlarge the contact area between the negative electrode and the solid electrolyte, allowing the battery to be charged faster.

Batteries produced like this could theoretically operate at normal ambient temperatures, says Semih Afyon, a former research scientist in Rupp's group, now a professor at the Izmir Institute of Technology in Turkey. But they actually work best at 95°C and above. "The lithium ions can then move around better in the battery," says Afyon.

This characteristic could be put to use in battery storage power plants, which store excess energy and deliver it later as needed. "Today, the waste heat that results from many industrial processes vanishes unused," says Afyon. "By coupling battery power plants with industrial facilities, you could use the waste heat to operate the storage power plant at optimal temperatures."

"Many of today's solid-state battery research projects focus on improving the electrolytes," says Afyon. However, there are few studies such as this one, in which the scientists assembled an entire solid-state battery – using methods also used in industrial production – and tested it.

"In this work we have for the first time built a whole lithium-ion battery with a solid lithium garnet electrolyte and a solid minus pole made of an oxide-based material. Thus, we have shown that it is possible to build whole batteries based on lithium garnet," says Rupp. As well as producing batteries that can operate at higher temperatures, the solid electrolyte could also allow the development of thin-film batteries that can be placed directly on silicon chips.

"These thin-film batteries could revolutionize the energy supply of portable electronic devices," says Rupp. She and her team will pursue this approach in further research. To this end, they have collaborated with industrial partners, as well as with the Paul Scherrer Institute and with Empa, both in Switzerland. The immediate next step for Rupp and her team is to optimize the battery, with a focus on further increasing the conductivity of the electrode-electrolyte interface.

This story is adapted from material from ETH Zurich, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.

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